In the perfect eye, an incoming beam of light is focused through the cornea and through the crystalline lens in a way that causes all of the light from a point source to converge at the same spot on the retina of the eye, ideally on the fovea area of the retina. This convergence occurs because all of the optical path lengths, for all light in the beam, are equal to each other. Stated differently, in the perfect eye the time for all light to transit through the eye will be the same regardless of the particular path that is taken by the light.
Not all eyes, however, are perfect. The consequences of this are that light path lengths through the eye become distorted and are not all equal to each other. Thus, light from a point source that transits an imperfect eye will not necessarily come to the same spot on the retina and be focused.
As light enters and passes through an eye it is refracted at the anterior surface of the cornea, at the posterior surface of the cornea, and at the anterior and posterior surfaces of the crystalline lens, finally reaching the retina. Any deviations that result in unequal changes in these optical path lengths are indicative of imperfections in the eye that may need to be corrected. For example, many people are near-sighted because the axial length of their eyes are “too long” (myopia). As a result, the sharp image of an object is generated not on the retina, but in front of or before the retina. Hyperopia is a condition where the error of refraction causes rays of light to be brought to a focus behind the retina. This happens because the axial length is “too short”. This condition is commonly referred to as farsightedness. Another refractive malady is astigmatism resulting from a refractive surface with unequal curvatures in two meridians. The different curvatures cause different refractive powers, spreading light in front and in back of the retina.
Other “higher order” maladies of interest for vision correction include coma and spherical aberration. Coma exists when an asymmetry in the optical system causes unequal optical path lengths in a preferred direction. For example, the image of an off-axis point object takes on a comet-like shape. For symmetrical systems, spherical aberration exists when rays at different radial heights from the optical axis focus at different axial locations near the retina. Whereas coma exists only in asymmetric systems, spherical aberration can exist in both symmetric and asymmetric systems. Other, even higher order, aberrations exist. However, studies have show that spherical aberration is one of the strongest higher order aberrations in the human visual system. Thus the retinal image may be improved if the spherical aberration is corrected according to known techniques.
Studies have also shown that there is a balance between the positive spherical aberration of the cornea and the negative spherical aberration of the crystalline lens in younger eyes. As one grows older, the spherical aberration of the crystalline lens becomes more positive, increasing the overall spherical aberration and reducing the image quality at the retina.
An intraocular lens (IOL) is commonly used to replace the natural lens of a human eye when warranted by medical conditions such as cataracts. In cataract surgery, the surgeon removes the natural crystalline lens from the capsular bag or posterior capsule and replaces it with an IOL. IOLs may also be implanted in an eye (e.g., in the anterior chamber) with no cataract to supplement the refractive power of the natural crystalline lens, correcting large refractive errors.
The majority of ophthalmic lenses including IOLs are monofocal, or fixed focal length, lenses that primarily correct refractive error. Most monofocal IOLs are designed with spherical anterior and posterior surfaces. The spherical surfaces of the typically positive power IOLs cause positive spherical aberration, inter alia. Thus, replacement of the crystalline lens with a typical monofocal IOL leaves the eye with positive spherical aberration. In real eyes with complex corneal aberrations, the eye following cataract surgery is left a with finite number of complex lower and higher order aberrations, limiting the image quality on the retina.
Some examples of attempts to measure higher order aberrations of the eye as an optical system in order to design an optical lens include U.S. Pat. No. 5,062,702 to Bille, et al., U.S. Pat. No. 5,050,981 to Roffman, U.S. Pat. No. 5,777,719 to Williams, et al., and U.S. Pat. No. 6,224,211 to Gordon.
A typical approach for improving the vision of a patient has been to first obtain measurements of the eye that relate to the topography of the anterior surface of the cornea. Specifically, the topography measurements yield a mathematical description of the anterior surface of the cornea. This corneal surface is placed in a theoretical model of the patient's eye with an IOL replacing the crystalline lens. Ray-tracing techniques are employed to find the IOL design which corrects for the spherical aberration of the cornea. Ideally, if implanted with this custom IOL, the patient's vision will improve.
Recently, Pharmacia Corp. (Groningen, Netherlands) introduced a posterior capsule intraocular lens having the trade name TECNIS (Z9000) brand of Silicone IOL. The TECNIS lens has a prolate anterior surface, which is intended to reduce spherical aberrations of the cornea. This lens may be designed using methods described in U.S. Pat. No. 6,609,793 and PCT publication WO 01/89424, both to Norrby, et al. The methods in these publications involve characterizing aberrant corneal surfaces as linear combinations of Zernike polynomials, and then modeling or selecting an intraocular lens which, in combination with a characteristic corneal surface, reduces the optical aberrations ocular system. The lenses resulting from these methods may be continuous aspherical surfaces across the entire optical zone and may be used to reduce spherical aberrations of the eye by introducing negative spherical aberration to counter the typically positive spherical aberration of the cornea. In these lenses, there may be a single base curve on which the aspheric surface is superimposed. As reported by J. T. Holliday, et al., “A New Intraocular Lens Designed to Reduce Spherical Aberration of Pseudophakic Eyes,” Journal of Refractive Surgery 2002, 18:683-691, the Technics IOL has been found to be to improve visual contrast sensitivity at a frequency up to 18 cycles/degree.
The TECNIS brand of lens generally requires precise positioning in the capsular bag to provide improved optical quality over a spherical IOL (c.f., “Prospective Randomized Trial of an Anterior Surface Modified Prolate Intraocular Lens,” Journal of Refractive sugery, Vo. 18, Nov/Dec 2002). Slight errors in decentration (radial translation) or tilt (axial rotation) greatly reduces the effectiveness of the lens, especially in low-light conditions, thus making the task of the surgeon more difficult. Furthermore, shrinkage of the capsular bag or other post-implantation anatomical changes can affect the alignment or tilt of the lens along the eye's optical axis. It is believed that the “typical” magnitude of decentration resulting from the implantation of an intraocular lens in an average case, and factoring in post-implantation movement, is less than about 1.0 mm, and usually less than about 0.5 mm. Most doctors agree that decentration of an IOL greater than about 0.15 to approximately 0.4 mm is clinically relevant (i.e., noticeably affects the performance of the optical system, according to those skilled in the art). Similarly, the “typical” magnitude of tilt resulting from the implantation of an intraocular lens in an average case, and factoring in post-implantation movement, is less than about 10 degrees, and usually less than about 5 degrees. Therefore, in practice, the benefits of the TECNIS brand of lens may be offset by its apparent drawbacks in the real world.
In view of the above, there remains a need for an intraocular lens that corrects for spherical aberrations in a variety of lighting conditions and is less sensitive to non-optimal states such as decentration and tilt of the IOL.